Specimens of finfish and shellfish species were collected fortnightly from three selected sites in the Ennore Creek of Chennai coast from September 2018 to August 2019 and August 2022 to July 2023 by employing gillnet. A total of 49 species of finfish and 12 species of shellfish were recorded during the study period. The finishes recorded were found to belong to 11 orders with the dominance of the order Perciformes (16 families, 27 species), followed by Siluriformes (2 families, 3 species), Beloniformes (2 families, 5 species), Clupeiformes (3 families, 4 species), Carangiformes and Acanthuriformes (each 1 family, 2 species), Elopiformes (2 families, 2 species), Scombriformes, Mugiliformes, Gonorynchiformes and Pleuronectiformes (each 1 family and 1 species) (Fig 2 and 3). The IUCN status of recorded species was classified mostly as Least Concern (77.6%), followed by Not Evaluated (16.3%) and Data Deficient (6.1%) (Fig 4). The majority of the recorded shellfish (83.33%) were found to be commercially important.
The IUCN status of all the recorded shellfish species was classified as Not Evaluated (100%). However, only a small number of finfish and shellfish species were found in the current study, which may be related to pollution at the study site, as proposed by
Plafkin et al., (1989). The current investigation concurs with
Plafkin et al., (1989) in that a mere 49 finfish species and 12 shellfish species have been identified. The remaining fish species were uncommon in their occurrence
(Goldin and Athalye, 2012). Our research is consistent with that of
Azzuro et al., (2011), who found that sewage significantly altered the levels of specific species and the fish assemblage in Mediterranean waterways.
Species diversity (H'
)
The spatial variation in Shannon-Wiener diversity (H'), value is given in Table 1. The highest (H') value was observed at S
3 (4.022) and the lowest in S
2 (3.917) during 2022-23. On comparing the seasons, the highest (H') value was observed in post-monsoon (4.027) and the lowest in monsoon (3.918) during the year 2022-23. The highest value (4.022) was observed in S
3 and the lowest value (3.917) was recorded in S
2. Similar results were observed in the Vellar estuary (4.52-5.81) by
Murugan et al. (2014) and
Nandini and Milton (2019) in the Adyar estuary (3.262-3.327). Observations were made by
Bharadhirajan et al. (2015), at Coleroon Estuary, Murugan
et al. (2014), at
Vellar Estuary and Pavinkumar (2014), at Korampallam-thermal Estuary.
The diversity of fishes was calculated for each station following Shannon- Wiener index using the following formula:
Species richness (d)
Species richness for each station was calculated following Margalef index (d) using the formula:
The spatial variation in Margalef species richness (d) among different sites of Ennore is given in Table 2. The highest value was observed in S
1 (9171) and the lowest in S
2 (8.875) during the year 2022-23 (Table 1). The seasonal variation in Margalef species richness (d) is given in Table 2. The species richness “d” ranged from 8.718 to 10.06. Our study confirms with the study carried out by
Plafkin et al. (1989) who opined that a community becomes more dissimilar as the stress increases and accordingly species diversity decreases with poor quality. The calculated d values of the present study lie more or less similar to that of
Murugan et al., (2014) for the diversity of fishes of the
Vellar Estuary and Pavinkumar (2014), for the diversity of fishes of the Korampallam-Thermal estuary. Furthermore, when comparing the two years, 2022-23 exhibited higher Margalef species richness (10.86) than 2018-19 (10.75). This suggests a potential increase in species richness over time in the Ennore Creek region. However, the values of d were recorded for the ichthyofaunal diversity of the Thamirabarani River by
Mogalekar (2019).
Taxonomic diversity (∆)
This is a measure of average taxonomic distance between any two individuals chosen at random belonging to separate species. It was calculated using the following formula:
∆ = [∑ ∑ i< j wij xi xj] / [N (N-1)/2]
The estimated taxonomic diversity (∆) values for the sampling stations are presented in Table 2. The highest taxonomic diversity value was observed at S
2 (85.29) during 2018-19 and the lowest value at S3 (84.24). During the year 2022-23, the highest value (83.96) was observed at S
3 and the lowest at S
2 (83.64). The present seasonal taxonomic diversity values (83.33-85.37) are almost similar to the values reported by
Jesintha et al., (2022) for the finfish and shellfish diversity of Pilicat Lake. Higher values of taxonomic diversity indices suggest that, on average; the species in the assemblage are not closely related, being higher biodiversity
(Patricio et al., 2009) shown in shade plots (Fig 5 to 7).
Species evenness (J')
Evenness was calculated following Pielou’s evenness (
j') using the formula:
J' = H'/log2S or H'/ln2S
Pielou’s evenness (J') can be used to compute evenness, which describes how uniformly the individuals in a community were allocated among the various species. The highest value was observed at S
3 (0.9784) and the lowest was S
2 (0.9759) during 2018-19. According to
Clarke and Warwick (2001), as the dominance of individual species increases, the level of species evenness decreases (Fig 5 to 7). The values of species’ evenness vary from 0 to 1, where a value of zero represents no evenness and a value of one represents complete evenness. The above concept holds well in the present study. Pielou’s evenness calculated in the present study was relatively higher in the range of 0.9568-0.9801. This indicates that the distribution of individuals among species in the community was more uniform in 2018-19 compared to 2022-23 (Table 3).
Mogalekar (2019) reported the average. The evenness value calculated for Pulicat Lake is higher which shows that the fish communities in the lake are not under stress
(Jesintha et al., 2022).
Taxonomic distinctness index (D*)
Taxonomic distinctness (D*) was calculated using the following formula:
D* = [S S i< j wij]/[S(S - 1)/2]
Spatial variation in average taxonomic distinctness (D*) is represented in Table 2. The comparison between the two years revealed that 2018-19 (86.66) had a slightly higher average taxonomic distinctness than 2022-23 (86.24). However, our present results are contrary to the taxonomic diversity index of freshwater fish in Guangxi, China, which falls within the range of 42.8 to 43.2
(He et al., 2022), showing that the diversity of fish species in the Ennore Creek is relatively good.
Variation in taxonomic distinctness index (D+)
Average taxonomic distinctness index (D+) was calculated using the formula:
D+= [S S i< j (wij - D+) 2] / [S(S-1)/2]
The spatial variation in taxonomic distinctness (D+) among the studied sampling sites (Table 1) is in the following ascending order. The study also examined the variation in taxonomic distinctness (D+) for each year and found that the values for 2018-19 and 2022-23 were both equal, with a value of 87.08.
Jiang et al. (2020) assessed the effects of anthropogenic impacts (loss of river-lake connectivity and deterioration of water quality) based on the presence/absence data of fish assemblages from the floodplain lakes in the Yangtze River Basin, China.
Total phylogenetic diversity (sPhi+)
Total phylogenetic diversity (sPhi+), which vouches safe for the taxonomic breadth for the fishes present in various sampling stations, was calculated by finding out cumulative branch length of the full taxonomic tree drawn using the Linnaean classification.
The total phylogenetic diversity (sPhi+) had the highest value at S1 and S3 (3833) for the two corresponding years 2018-19 and 2022-23 and the lowest value at S
2 (3533) during the years 2018-19 and S
2 (3617) during the year 2022-23 (Table 1). In a healthy environment, due to rich faunal assemblages, (taxonomic breadth) the total phylogenetic diversity and average phylogenetic diversity are always higher
(Khan et al., 2005; 2008). The values obtained in the present study are higher than those obtained by
Pavinkumar (2014) for the diversity of fishes in the Korampallam Thermal, Punnaayal and Manakudy estuaries;
Murugan et al. (2014) for the diversity of fishes in Vellar estuary.
K-dominance curve
Chandran et al., (2022), stated that K-dominance curve was plotted season-wise. Cumulative relative abundances were higher in monsoon followed by winter and summer. The month-wise D dominance plot showed that the cumulative abundance was rich during August, 2019 and September, 2019 and poor during February, 2020. The spatio-temporal variation in the K dominance plot among the sampling stations indicated that the flow of cumulative abundances is the flow of station 1>2>3>4>5. The seasonal variation recorded for Gorai Creek resulted in high during monsoon, 2019 and less during summer, 2020. To examine the dominance of individual species across stations, dominance plots were created by ranking the species in decreasing order of abundance. Among the sampling sites, in 2022-23, S
2 had a relatively more significant cumulative abundance than the other sites and in 2018-19, no significant difference was observed in the species abundance. The Spatio-temporal variation and the seasonal variation in the dominance plot among the sampling sites at Ennore Creek, Chennai are presented in Fig 8 to 10.
Analysis of similarity
Bray-Curtis similarity helps to quantify the compositional similarity between two sites/ seasons/ years, based on counts at each location. The calculated values are presented in Fig 11 to 13. While analyzing the spatial variability in the BC similarity index for individual sampling sites, it was higher (94.24%) between S
2-22 and S
2-18; and 82.67% between S
2-18 and S1-18 with high positive Cophenetic correlation (0.8473). Contrary to this, the lowest similarity (64.24%) has been observed between S
2-22 and the combination matrix of other sites
(Clarke and Warwick, 1999).
nMDS plot
According to our dissimilarity analysis, 2017 and 2018 shared 80% of the species recorded in the study, which is also consistent with the similar temperatures and diversity values recorded in both years. In addition, average taxonomic distinctness also shows 2017 and 2018 as highly similar years. According to
Ruiz-Campos et al., (2010), the fish species that inhabit the intertidal rocky areas of the West coast of the Baja California peninsula have a greater affinity for the Mexican and Cortes zoogeographic provinces, with the family Pomacentridae recording the highest number of species, particularly
A. declivifrons and
A. troschelii. This is consistent to that reported by
Cota-Ortega et al., (2022), who also mentioned these species as the most abundant. A simulation test performed on average taxonomic distinctness using a funnel showed all the values for spatial abundance for two-year sampling (2018-19, 2022-23) falling within the 95% confidence limit. Values for the spatial abundance fell exactly on the mean (D+, 87.08; l+, 139.15), while for the S2-22 it varied slightly from the mean (D+, 87.28; +, 140.78) with significance of (D+) 59.3% and (+) 63.3%, respectively. A simulation test performed on average taxonomic distinctness using a funnel showed all the values for seasonal abundance for two-year sampling (2018-19 and 2022-23) falling within the 95% confidence limit (Fig 14 to 17).
Physico-chemical properties of seawater
Water is one of the most important compounds to the ecosystem. Physico-chemical and micro-biological characteristics may describe the quality of water
(Priyanka et al., 2019). The marine environment, as a complex system is mainly influenced by various physical, chemical and biological processes. Estuaries aid creeks play an important role as nursery grounds for fishes and prawns. However, they are vulnerable to anthropogenic activities, as they are being used as dumping grounds for domestic and industrial wastes. Short term and long-term studies of the hydrological parameters can provide firsth and information about the chemical interactions taking place in an aquatic ecosystem
(Quadros et al., 2001).
The study reveals water quality parameters of Ennore estuary showed that concentrations of nutrients were above the coastal water level due to continuous discharge of domestic sewage and industrial effluents and the estuary is severely polluted (Table 4). The continues discharge of effluents to the estuarine ecosystem is vulnerable to all compartments of the food web. Awareness has been created if not immediate, definitely block or alert the input from industrial area. Estuaries are extremely exploited ecosystems, due to their proximity to major civilization throughout the globe. There is an urgent need to control or restore the discharge of domestic sewage and other industrial effluents to restore breeding ground of finfish and shell fish, secondary and tertiary productivity in the estuarine water body for the benefit of Chennai coast.
